The use of biocatalysts such as microbial cells or enzymes to make products is well known and has been practised for centuries in what has become known as biotechnology processing. Typically in biotech processes microorganisms are cultivated in a tank (also called a fermentation tank, or simply a fermenter) into which the substrates necessary for the microorganisms to make the product are added.
Such cultivation processes typically occur in aqueous solutions (also called fermentation liquids, fermentation broths, or simply broths) containing a variety of substrates such as carbon sources as well as nitrogen sources, phosphates, sulphates plus a wide variety of other components depending on the microorganism used and the products to be made. In many cases the generic name fermentation is used for such processes which may be carried out in the presence or the absence of oxygen or air.
In many cultivations, the microorganisms used require oxygen, and this must be added. Oxygen is typically added as a gas by pumping compressed atmospheric air into the fermentation broth. Pure gaseous oxygen or oxygen enriched air may also be used as an oxygen source. Cultivations, in which oxygen is added, are called aerobic fermentations or aerobic cultivations. In some cases, other substrates used by the microorganisms are added as gases to the solution, and may for example be carbon sources such as methane. Waste products are also formed during fermentation. One waste product that is normally produced in the largest amount is carbon dioxide.
The addition of gaseous substrates such as oxygen or methane to fermentation liquid is problematic, since the microorganisms cannot use the gases directly. The gases must therefore be dissolved in the fermentation broth, after which they are taken up by the microorganisms and used as an energy source and/or for microbial growth. A generic problem in fermentation processes is therefore to solubilise enough of the gaseous nutrients in the broth to satisfy the demands of the microorganisms, particularly if the population or concentration of microorganisms is large (and the demand for gaseous substrates is high), or if the fermentation temperature is high, which tends to lower the solubility of the gaseous substrates in the fermentation broth.
The transfer rate of substances from the gas phase into the liquid phase can be improved if very small bubbles are used, or if a higher pressure (i.e. the pressure in the headspace of the fermenter is above atmospheric pressure) is used in the fermenter, or if the temperature of the broth is reduced. Since microorganisms are sensitive living cells, significant reduction of the temperature is not possible without affecting the growth rate of the microorganisms.
A large amount of energy is typically used in conventional fermentation tanks to ensure that sufficient amounts of the gaseous substrates are dissolved in the fermentation broth. Even so, many fermentation processes are limited by insufficient transfer of gases into the liquid phase.
Conventional fermenters are tall stirred tanks in which the mixing of gases with the fermentation liquid is effected by means of stirrer blades placed centrally in the fermenter. The stirrer blades generate turbulence in the liquid, which means that gas, usually injected at the bottom of the reactor, will be dissipated in the liquid in the form of small fine gas bubbles. The gaseous substrates are added at the bottom of the tank and must be pressurised to overcome the hydrostatic pressure in the tank into which they are pumped. This compression of gases requires significant amounts of energy.
This type of reactor provides a relatively homogenous mixing, i.e. that about the same concentrations of gases and substrates will be found whether measuring at the top or at the bottom of the reactor. But the vigorous mixing in order to create small gas bubbles and ensure optimal mixing in the tank also requires the use of excessive energy and further implies a significant heating of the fermentation liquid. The excessive use of energy renders this type of reactor uneconomical, especially for cheap products such as microbial cells, which are currently sold as animal food or fish food.
Other fermenter types have also been designed with the intention of reducing energy consumption for mixing but still ensuring sufficient mass transfer of gases to the liquid phase. These fermenters are often called air lift fermenters, jet loop fermenters or U-loop fermenters.
Different types of air lift reactors have been designed in order to avoid the mechanical stirring. The majority of these reactors are so-called loop reactors having two sections: an up-flow part and a down-flow part, which are interconnected at both ends. Gases are supplied as small bubbles at the bottom of the reactor in the up-flow part usually in a nozzle arrangement. The bubbles mix with the liquid, whereby the total density is reduced and the gas-liquid mixture ascends while being displaced by new liquid emerging from the down-flow part. The gas-liquid mixture moves up through the up-flow part of the reactor and releases gas bubbles at the top. Then, the liquid descends down through the down-flow part. In order to obtain a long residence time for the gas bubbles in the liquid. Airlift reactors are conventionally very tall slender reactors, and the gas must be supplied at a high pressure for overcoming the hydrostatic pressure at the bottom of the reactor. If the gas is air, this implies the use of compressors. Compression of air usually requires significant amounts of energy.
Airlift reactors have a relatively poor exploitation of the injected gas. Typically only 20-40% of the oxygen gas is utilized. It is often difficult to obtain a good and quick release of the gas bubbles from the fermentation liquid at the top of the reactor and separation of the gas phase thus produced (which may be rather foaming) from the liquid phase before the liquid flows into the down-flow part of the reactor. The gas phase, including significant amounts of waste gases from the fermentation, e.g. CO2, is thus entrained in the broth, is then re-dispersed in the broth, which may lead to a reduced solubilisation of the substrate gases added to the fermenter.
The U-shape reactor has a simple design and is constructed with a view to provide non-compressed or nearly non-compressed substrate gas injection in combination with a long residence time for the gases and thus a high degree of exploitation of the injected gases. The top of the reactor is designed to achieve a good separation of gases and liquid.
In principle, the U-shape reactor is also a loop reactor. However, contrary to conventional loop reactors, the liquid circulation is effected by means of an in-line pump. The pump may be of the propeller pump type, wherein the propeller blades are designed for pumping a mixture of liquid and gas. Using pumps instead of injected compressed air or gases to create liquid circulation reduces the overall energy consumption during fermentation.
The substrate gases may be introduced at different locations in the U-shape loop. Typically they will be supplied at the upper end of the down-flow part of the loop. By introducing the substrate gases at the upper end of the down-flow part of the loop a nearly non-compressed injection is obtained, since the gases only have to overcome a hydrostatic pressure of some few meters. The gases may be introduced by means of gas dispensers providing for a distribution across the entire cross-section of the down-flow part of the loop. Fine dispersion of the gases in the liquid is effected by means of static mixing elements placed immediately downstream of the gas injectors (the mixing elements may be of e.g. Sulzer manufacture). The liquid flow in the down-flow part of the loop must be sufficiently high for carrying all the injected gas through the static mixers. In the static mixers, the gas phase is broken down into a large number of small gas bubbles, which are dispersed uniformly in the liquid. The bubbles are carried along with the liquid flow down through the down-flow part of the loop to its lower end and further on through a U-bend to the up-flow part of the loop. The gas bubbles may be re-dispersed (e.g. by means of a plurality of static mixing elements provided in both the down-flow and the up-flow part of the reactor) several times in the liquid.
The in-line pump is normally placed adjacent the U-bend, partly because it then assists in producing a re-dispersion of the gas in the liquid, and partly because it is practical to have it placed at the bottom of the fermenter.
The top of the fermenter is designed so that the up-flow part of the loop, via a bend, is passed horizontally and tangentially into the side of a widening of the upper end of the down-flow part of the loop. This particular construction feature assists in yielding a good separation of liquid and gas bubbles, as centrifugal forces act in the bend, and in the very widening of the upper end of the down-flow part of the loop a vigorous circulation of the liquid with corresponding accompanying centrifugal forces arises, which also brings about separation of liquid and gas bubbles. Thereby, one of the great problems associated with airlift reactors, viz. separation of the gas and liquid phases, is elegantly solved.
Furthermore, the U-shape reactor provides for a long contact time between the gas and liquid phases, as the injected gas is present both in the down-flow part and in the up-flow part of the loop. This means that an essentially higher utilization of the gas is obtained compared with conventional airlift reactors and stirred tanks.
Gas bubbles in liquids have a tendency to fuse together to larger bubbles (coalesce). This tendency contributes to making conventional airlift reactors ineffective inasmuch as the bubbles become larger and larger upward through the up-flow part, partly due to coalescence and partly due to a reduced hydrostatic pressure. In the U-shape reactor, this tendency in the up-flow part is counteracted by providing static mixers appropriately spaced apart at distances which depend on the medium applied. In the down-flow part, the increasing hydrostatic pressure counteracts the tendency to increased bubble sizes. To the extent that this effect cannot balance the coalescence of the gas bubbles, static gas mixers are provided for re-dispersion of the gases by static mixers.
The amount of gas, which may advantageously be dispersed in the liquid, depends on the hydrostatic pressure. In the case of tall reactors, it will therefore be advantageous to have several locations for the introduction of gases in the down-flow part. The only requirement with respect to the gas inlets is that at least one static mixing element is placed immediately after each inlet for dispersing the gas in the liquid.
Examples of airlift fermenters may be seen in EP 306466 A or U.S. Pat. No. 5,342,781 A.
Examples of U-loop fermenters of the above mentioned type are disclosed in EP 185407 A, EP 418187 A, EP 1183326 B and WO 03/016460 A1.
In EP 1183326 B and WO 03/16460 A1, which are considered to describe the closest prior art of the present invention, the fermentation process is controlled using sensors and analyzers in the fermenter delivering signals to data processing units, which then control the addition of gaseous substrates and other process parameters.
In WO 03/16460 A1 a substantial part of the U-loop in the reactor is horizontal in order to overcome problems a rising from an increasing hydrostatic pressure. This construction of the fermenter may result in gas pockets building up in the top of the horizontal part of the U-loop, and thus, reducing the liquid volume in the fermenter and reducing the overall productivity of the fermenter.
Nevertheless, the transfer of gases between the gas phase and the liquid phase in such fermenters is still too poor for producing inexpensive products in a fermentation process.
Gas mass transfer may be improved by increasing the pressure in the headspace (i.e. the space above the liquid surface in the fermenter) e.g. up to 1 bar or more above atmospheric pressure. This increase in pressure in headspace increases the pressure in the entire fermenter.
One problem with increasing the pressure in the fermenter is that the release of waste gases from the fermentation liquid is reduced when increasing the pressure. The energy employed for pressurising the headspace cannot be recovered. If the carbon source is supplied as a gas, e.g. methane, then explosive gas mixtures in the headspace may arise more easily when the pressure is increased.
One of the waste products produced in large amounts in fermentation processes is carbon dioxide (CO2). All microbial cells produce carbon dioxide, which passes out of the cell and into the fermentation liquid. The carbon dioxide must then be transferred to the gas phase before it can be released from the fermenter. The solubility of the gases, including carbon dioxide, in the fermentation liquid is increased by increasing pressure. Thus, a higher pressure in the fermenter will reduce the release of carbon dioxide to the gas phase in the fermenter, resulting in a higher concentration of carbon dioxide in the fermentation broth. A high concentration of carbon dioxide in the fermentation liquid causes a reduced productivity of the cells in the fermenter and thereby a reduced overall productivity in the fermentation process.
Although the above-mentioned references also deal with optimisation of fermenters and methods of fermenting in which the gases supplied to the fermenter are more expensive gases, such as oxygen enriched air, pure oxygen and/or methane, there is still a need for further improvement of the overall productivity of the fermenters and the fermentation processes and especially further improvement of the utilisation of the substrate gases added to such fermentations.
No fermenter, fermenter or tank suitable for cultivation of living cells has previously been described that permits the pressure to be increased at will in certain parts of the fermenter, while the pressure is substantially equal to or even lower than atmospheric pressure in other parts of the reactor.